THE ORIGIN OF NEW CELLS AND TISSUES 199 



the muscles, and also after injury to the muscles. From these spindle 

 cells the new muscle fibres develop in the same way as the muscle 

 cells of the embryo. 



Fraisse sums up the results of his studies of regeneration as fol- 

 lows : (i) Both in amphibians and reptiles, injured tissues can only 

 produce new tissues like themselves. The leucocytes assume only 

 the function of nutrition and of devouring the broken-down parts of 

 tissues. They never become fixed tissues neither connective tissue 

 nor any other sort. (2) All tissues are capable of regenerating them- 

 selves, either directly out of their differentiated elements, or out of a 

 matrix. As a matrix for the epidermis, there is the Malpighian layer 

 of the skin; for the central nervous system, the epithelium of the 

 central canal of the nerve-cord ; and for the musculature, the spindle 

 fibres. 



Fraisse also formulates the following general statements : (a) Re- 

 generation is neither a pure recapitulation of the ontogeny nor of 

 the phylogeny. The process is rather a hereditary one, with which 

 complicated adaptations of the tissues are often involved that fol- 

 low the laws of correlated development, (b) We cannot explain the 

 phenomenon of regeneration, as the result of wounding the tissues, 

 or as the outcome of an increase in the food supply, or as due to the 

 removal of a resistance to growth. Far more important are the prin- 

 ciples covered by the former paragraph, (a). 



Barfurth has studied in detail the regeneration of the tail in some 

 amphibia ; and his results, while not covering as much ground as do 

 those of Fraisse, yet give a more detailed account of the origin of the 

 new tissues. Barfurth's results on triton and siredon are not essen- 

 tially different from those of Fraisse. In the tadpole of the frog, Bar- 

 furth finds that the notochord regenerates from the sheath of the old 

 notochord. In the larval urodele, he finds that the new notochord 

 arises as in the tadpole, and not from the skeletal sheath, as Fraisse 

 maintains. In very young larvae of siredon the chordal cells them- 

 selves seem to give rise to the cells of the new notochord. In older 

 larvae, in which the skeletal tissue is developed around the notochord, 

 regeneration takes place both from this tissue and also from the sheath 

 of the notochord. He concludes that in the regeneration of the new 

 notochord, and also of the skeleton, the origin of the cells depends 

 upon the developmental stage of the supporting tissues. 



In regard to the regeneration of the muscles, Barfurth comes to 

 the following conclusions : In very young larvae of siredon, the de- 

 generative changes in the muscle cells are often very slight. Regen- 

 eration takes place by growth from and the displacement of the old 

 muscles. During this time bud-like terminal and lateral formations 

 occur in the muscle fibres. These outgrowths contain nuclei and 



